[Fix] Warning filter in fluctuating LB test

The fluctuating LB test used an old syntax of the pytest module to check for warnings. This now triggers an error and lets the test fail. With this MR new syntax suggested by the pytest package is used.

Furthermore, fix of some warnings

lbmpy_tests/test_fluctuating_lb.py

 """Tests velocity and stress fluctuations for thermalized LB"""
 
+import warnings
+import pytest
 import pystencils as ps
+
+from pystencils import get_code_str
+from pystencils.backends.simd_instruction_sets import get_supported_instruction_sets, get_vector_instruction_set
+from pystencils.cpu.cpujit import get_compiler_config
+from pystencils.enums import Target
+from pystencils.rng import PhiloxTwoDoubles
+
 from lbmpy.creationfunctions import *
 from lbmpy.forcemodels import Guo
 from lbmpy.macroscopic_value_kernels import macroscopic_values_setter
@@ -8,13 +17,6 @@ import numpy as np
 from lbmpy.enums import Stencil
 from lbmpy.moments import is_bulk_moment, is_shear_moment, get_order
 from lbmpy.stencils import LBStencil
-from pystencils.rng import PhiloxTwoDoubles
-
-import pytest
-from pystencils import get_code_str
-from pystencils.backends.simd_instruction_sets import get_supported_instruction_sets, get_vector_instruction_set
-from pystencils.cpu.cpujit import get_compiler_config
-from pystencils.enums import Target
 
 
 def _skip_instruction_sets_windows(instruction_sets):
@@ -294,13 +296,11 @@ def test_vectorization(data_type, assume_aligned, assume_inner_stride_one, assum
                                    )
 
     if not assume_inner_stride_one and 'storeS' not in get_vector_instruction_set(data_type, instruction_set):
-        with pytest.warns(UserWarning) as warn:
+        with pytest.warns(UserWarning) as pytest_warnings:
             ast = ps.create_kernel(collision, config=config)
-            assert 'Could not vectorize loop' in warn[0].message.args[0]
+            assert 'Could not vectorize loop' in pytest_warnings[0].message.args[0]
     else:
-        with pytest.warns(None) as warn:
-            ast = ps.create_kernel(collision, config=config)
-            assert len(warn) == 0
+        ast = ps.create_kernel(collision, config=config)
     ast.compile()
     code = get_code_str(ast)
     print(code)